The present invention relates generally to a method and apparatus for detecting the presence of particles on a material and more particularly to a method and apparatus for detecting the presence of particles on a material using the scattered light principle.
Although the invention is especially useful in detecting particles on a material such as the surface of a semiconductor wafer, it is to be understood that the invention is applicable for use in connection with other materials, objects or bodies.
In the prior art there are a variety of ways in which to detect and measure the number and sizes of particles on a semiconductor wafer, for the purpose of rejecting those wafers which have on their surface one or more particles above certain sizes, e.g. from 1 to 20 microns, or for those having on their surface an excessive number of particles.
One of the most prevalent methods employs the human operator using a light field/dark field microscope. Using the eye, the operator actually counts the number of particles and also identifies the size of the particles, such as those between 1 to 20 microns, and then rejects those wafers which have an excessive number of particles or those having particles of or above a certain size. This method is without doubt highly inaccurate, and very expensive both in terms of wages for the human operator, and in terms of the number of rejects both after the inspection and after production of the chips (when an erroneously passed wafer is found to have an electrical defect, e.g. short circuits, because of the presence of contaminant particles).
In U.S. Pat. No. 4,377,340 to G. P. Green etc., there is disclosed a method and apparatus for detecting and measuring the number and sizes of impurities on the surface of a material, such as a semiconductor wafer, wherein a beam of high intensity collimated light is directed onto the surface at normal incidence in the absence of any extraneous light, through a collimating mirror and employing a point source, whereat the particles will scatter the light, and wherein the surface is viewed by a highly light sensitive TV camera which picks up scattered light only and displays the scattered light on a viewing screen.
In IBM Technical Disclosure Bulletin Volume 2, No. 10, pages 1672-1673, dated March, 1970, there is disclosed a system for detecting repeated geometric defects on a reflecting surface in which a collimated light beam strikes the surface being examined at a finite angle of incidence. Light scattered back along the same axis as the angle of incidence is directed through a telescope to a photomultiplier tube.
In IBM Technical Disclosure Bulletin Volume 21, No. 6, pages 2336-2337 dated November, 1978, there is disclosed a system for detecting defects on wafers wherein light from a plurality of ring light sources impinges on the wafer at an oblique angle to the wafer surface and wherein light scattered upward from the surface at right angles thereto is fed by a lens system into a broad band array detector.
An attachment has been reported for use with a microscope of the type having a 45.degree. mirror mounted on the microscope tube and a lateral opening along the length of the microscope tube to enable the entire mirror surface to receive a collimated beam of light directed to the mirror from a source next to the microscope tube. The attachment is a single optical element made of glass which is mounted on the tube in axial alignment therewith. The optical element contains a first curved reflective surface for causing the light reflected from the 45.degree. mirror to be made converging and sent out in all directions and a second curved reflective surface for redirecting the converging light beam so that it comes to focus underneath the microscope tube from all directions. As can be appreciated, any imperfections or bubbles in the glass will produce scattering as the light beam passes from the first reflective surface through the optical element to the second reflective surface.
It is an object of this invention to provide a new and improved method and apparatus for use in detecting the presence of particles on a surface using the light scattering principle.
It is another object of this invention to provide a method and apparatus as described above in which the surface being examined is illuminated from all directions with a beam of light incident on the surface at an oblique angle and forming a small spot on the surface.
It is still another object of this invention to provide a novel optical arrangement for illuminating a spot on a surface with light from all directions and then detecting light scattered from any particles on the surface.
It is yet still another object of this invention to provide a method and apparatus as described above which utilizes mirrors which are only first surface reflecting.
It is a further object of this invention to provide a method and apparatus as described above in which the scattered light is imaged through a continuous tube onto a light detector.
It is another object of this invention to provide a method and apparatus as described above wherein unwanted scattered light is reduced to a minimum.
It is still another object of this invention to provide a method and apparatus as described above in which a sample being tested is illuminated an area at a time.
It is another object of this invention to provide a system for examining surfaces useful in applications requiring illumination by an ultraviolet light beam.
It is a further object of this invention to provide a system designed especially for use in dark field illumination applications.
The foregoing and other objects and advantages will appear from the description to follow. In the description, reference is made to the accompanying drawing which forms a part thereof, and in which is shown by way of illustration, a specific embodiment for practicing the invention. This embodiment will be described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that structural changes may be made without departing from the scope of the invention. The following detailed description is, therefore, not to taken in a limiting sense, and the scope of the present invention is best defined by the appended claims.